A photovoltaic cell which is also called a solar cell is a semiconductor device capable of converting light into electricity. If the photovoltaic cell is exposed to light, it generates a voltage, thereby inducing an electron flow. In this case, the magnitude of the electron flow is proportional to the collision strength of light on a photovoltaic cell junction formed on the cell surface.
Typical photovoltaic cells include a silicon wafer photovoltaic cell and a thin film photovoltaic cell. The silicon wafer photovoltaic cell uses a photoelectric transformation element which is prepared using a single crystal or polycrystalline silicone ingot, and a photoelectric transformation element used in the thin film photovoltaic cell is deposited on a substrate or a ferroelectric using a method such as sputtering or deposition.
Since both of the silicon wafer photovoltaic cells and the thin film photovoltaic cells are brittle, such photovoltaic cells require a support member having resistance to loads. The support member may be a light-transmissive top layer disposed on a photovoltaic cell, or a bottom layer disposed at a rear surface of the photovoltaic cell.
The bottom layer disposed at the rear surface of the photovoltaic cell is generally in the form of rigid back skin. Various materials which can be applied onto such a bottom layer are known, and examples of the materials include a ferroelectric such as glass; a metal foil such as aluminum foil; organic fluororesin; or a polyester polymer film on which the fluororesin or the metal foil is laminated.
A structure in which an organic fluororesin sheet is laminated on a polymer substrate, such as poly (ethylene terephthalate), or a structure in which a coating layer is formed on the polymer substrate by coating the organic fluororesin is most frequently used. Such materials may be applied alone to a photovoltaic module or also may be applied after the materials are coated with a silicon or oxygen material such as SiOx.
However, currently used fluororesin sheets have low mechanical strength as well as low adhesive strength to an encapsulant or polymer base of a photovoltaic module, and thus have problems with durability when used for a long period of time. Also, the fluororesin sheet has poor processability and is very expensive. Also, a polymer base coated with the fluororesin has poor adhesive strength to the polymer base since the fluororesin itself shows poor coatability.
Also, a technique of preparing a laminated sheet using a polyolefin has also been known in the art. However, such a polyolefin has insufficient properties such as heat resistance, lightfastness and weather resistance, and thus is not suitable for use as a bottom layer required to have long-term reliability.